Melt the chocolate slowly in a heatproof bowl over a pan of barely simmering water.
Remove the bowl from the heat and add the butter. Stir until melted and combined.
Add the egg yolks one by one, stirring constantly.
Add either the rum, brandy or espresso coffee (depending on your preference).
Whip the egg whites until soft peaks form when the whisk is removed, then gently fold them into the chocolate mixture.
Transfer the mousse to small cups or ramekins and refrigerate overnight, or until ready to serve.